1. Field of the Invention
This invention relates to processes for preparing aqueous resin dispersions wherein the dispersed resin comprises more than 50%, by weight, of acrylic monomers, and more particularly to preparing such dispersions by monomer-addition processes.
2. Description of the Prior Art
In monomer-addition processes for preparing acrylic resin dispersions a homogeneous liquid monomer phase, containing the acrylic monomers and, optionally, smaller amounts of other monomers and other materials dissolved therein, is added to a prepared aqueous phase in the presence of a free-radical initiator and an emulsifier under conditions appropriate for polymerization.
A monomer-addition process for preparing acrylic resin dispersions is disclosed in German Pat. No. 2,135,073. During the polymerization, a monomer mixture is gradually added to an aqueous phase in which an initiator and an emulsifier are dissolved. In this process troublesome amounts of coagulated material can easily form, especially when the dispersion so prepared has a high solids content.
It is known from German Pat. No. 1,119,513, to add aqueous solutions of initiators and emulsifiers slowly, simultaneously with the monomer phase, but separate therefrom, in processes of this type. However, these processes also have a tendency to form coagulated material.
In the preparation of aqueous resin dispersions based on vinylaromatic monomers, it is known from British Pat. No. 713,492 to dissolve the emulsifier in the monomer phase and to add this gradually, under conditions suitable for polymerization, to an aqueous phase which is free of emulsifiers. An aqueous initiator solution is added separately at the same time. These processes yield thin vinyl aromatic resin dispersions having very large particles which are unsuitable for many industrial purposes because of their low viscosity and large particle size.
According to the processes of emulsion polymerization used hitherto, e.g., emulsion addition processes, dispersions having limited solids content or a low content of carboxylic acid groups can be easily prepared, especially when large proportions of emulsifiers can be used.
On the other hand, difficulties arise in these processes, especially when small proportions of emulsifiers are used, when the solids content is increased to 55%, by weight, or above, or more than 6%, by weight, of water-soluble monomers are incorporated and it is desired to control the particle size at the same time. Highly concentrated dispersions are desirable because of their lesser weight and volume, relative to the resin content, and because of the lesser energy requirement in spray drying or drying of impregnated material.
Dispersions having a high proportion of units derived from water-soluble monomers incorporated therein are required, for example, as coating materials for drug dosage forms. They can be prepared by emulsion addition processes or the single pot process only with difficulty, since forming the monomer emulsion is difficult in itself. High concentrations of emulsifier are needed because of the requirement for forming very small latex particles. When latex particles of dissimilar chemical composition are formed incompatibilities arise and, consequently, coagulation.
Therefore, a need has continued to exist for a process of preparing aqueous acrylic polymer dispersions which avoids the problems of known processes for preparing these materials.